Flow through rail road freight cars are typically used for carrying bulk commodities in the form of ore, aggregate, granules, grain, ash or pellets. The cars typically have a containment structure, which may be a hopper, or an array of hoppers, that includes one or more entrances or hatches or intakes at the top, and one or more exits, outlets, or gates at the bottom. The lading, of whatever type it may be, is of a kind that may tend to flow somewhat like a liquid under the urging of gravity. Perhaps the most common example of this type of car is a center flow car of three or four hoppers.
Generally speaking, it is desirable for a center flow car to have a large internal volume. It is also generally desirable for that internal volume to correspond to the amount of intended lading that will tend to match the permissible gross rail load for that rail road car. The volume required to achieve this will depend on the density of the lading. It may not be desirable to fill the full potential volume of the car with a higher density lading if to do so would cause the car to exceed its allowable gross rail load, be it “100 Tons” i.e., 263,000 lbs GRL, “110 Tons”, i.e., 286,000 lbs GRL or “125 Tons” i.e., 315,000 lbs GRL.